There should be nothing in PHPExcel that's incompatible with PHP 5.4.7, it should work without issue: nor are there any known problems with cell content length until you start hitting Excel's own limits (32,767 characters since Excel 2003).
One obvious PHP settings that could cause issues is if the timezone hasn't been set (which now issues a warning in PHP whenever date functions are used)
Open the files in a text editor, and see if they contain any error messages at the top of the file before the stream of binary data.